CMake does not find includes / libraries

I want to use some third-party headers (or a library) in a project that uses CMake. But it does not find the headers (the library). Why does CMake not find it?

CMake's find routines look for headers and libraries at some specific places. This includes the PATH variable, and the default locations for installed software, e.g., for many Linuces /usr/bin. Additionally, it evaluates the CMake variable CMAKE_PREFIX_PATH.

You have two possibilities to help CMake finding the required files:

1. Check whether your software is properly installed. For self-compiled software, that's usually done by make install or similar. If you use packages (RPM or deb), they are in general installed and can be found with the PATH variable.

2. If you don't want or can install the software, add its path to the CMAKE_PREFIX_PATH variable. Either pass it to the CMake call cmake -DCMAKE_PREFIX_PATH=/path/to/software .. or edit/add the according field in the CMake-GUI.

You have to delete the CMakeCache.txt, otherwise CMake will not find the library, because it does not check but use the cached result. Re-run CMake and it should work.

Evaluation order

If you have multiple versions of a library on your system, add the one you want to use to the CMAKE_PREFIX_PATH as the variables gets evaluated prior to the system path variables.

Module-specific variables

Some modules offer specific variables like mylib_DIR or mylib_ROOT to indicate the search path. Its use is discouraged and they are only left for backwards-compatibility. New modules don't have these modules and commits adding such variables are rejected by the CMake developers.

Documentation

More details on how CMake searches files and in which order can be found in the documentation: https://cmake.org/cmake/help/latest/command/find_library.html